Modern power distribution systems use poly phase power lines for distributing electricity. A poly phase power line comprises a plurality, typically three, conductors, each conductor carrying a specified phase voltage. As is well known, a poly phase powder line may or may not have a neutral conductor which, if present, constitutes an additional conductor of the poly phase power line. Moreover, in addition to these conductors of a typical poly phase power line, there may or may not be a further conductor which carries the ground potential.
While a poly phase power line offers advantages for certain types of loads, e.g. electrical machines employing rotating magnetic fields, there are many electrical consumers which are not connected to all the phases available in a given poly phase power line. For many types of loads it is sufficient that the load is connected between two of the phases, or more typical, between one of the available phases and the neutral conductor. This wiring scheme is widely spread particularly in the low voltage networks used for supplying consumer appliances with electricity in the domestic domain. In Europe, the low voltage power distribution network has three wiring phases, each at a voltage of 220 Volt to 240 Volt to neutral, the three phases being spaced apart at an angle of 120°.
Particularly in the domestic domain, most electrical loads are connected between one of the three wiring phases R, S, T and the neutral conductor N, the particular phase R or S or T to which the load is actually connected, being insignificant for most types of single phase applications and loads, and therefore typically unknown. It has to be noted that there exists a variety of different naming conventions for the three wiring phases of a three phase power line. The naming convention R, S, T herein used shall not result in any loss of generality.
In some cases it is desirable to detect the wiring phase to which a given load is connected. For example, in a power line communication system that uses the existing power distribution network for telecommunication purposes, it may be highly desirable for the transmitter to know the wiring phase to which the receiver is connected, because it may be expected that the communication between the transmitter and the receiver via a power line is better if the transmitter and the receiver are connected to the same wiring phase than if the transmitter and the receiver communicates with each other across different wiring phases through capacitive or inductive cross talk between the wiring phases. If the meters communicate with other nodes in a remote metering system through power line communication, the knowledge of the phase to which the respective remote meter at the consumer premises are connected, is valuable information for optimising the communication performance of the remote metering system as a whole.
In an electricity metering system for metering the electric energy consumed by a plurality of consumers, there are other good reasons to detect the wiring phase of an electricity meter located inside or outside of the consumer premises. For example, a single phase or poly phase electricity meter may have been rewired by connecting its ground terminal to a wiring phase for supplying the consumer. The detection whether the wiring phase of the meter has been inverted, allows to judge whether the electricity meter has been rewired unlawfully or unintentionally such that the meter does not correctly measure the consumed energy.
From U.S. Pat. No. 4,626,622 it is known to identify an unknown wiring phase within a poly phase network by comparison of the unknown phase with a known reference phase of the poly phase network. The system comprises a first device connected to the reference phase at a first location and a second device connected to the unknown phase at a remote location. The first and second devices each comprise a modem for establishing a telephone connection between the two devices. The first device includes circuitry to produce a digital alternating signal representative of the phase of the alternating voltage of the reference phase. This representative signal is transmitted through the two modems and the telephone connection from the first to the second device. The second device includes a phase detection circuit for identifying the unknown phase by detecting the phase angle between the alternating voltage of the reference phase and the alternating voltage of the unknown phase. Although this known system allows detection of the wiring phase at the remote location relative to a reference phase at a reference location, it is not practical for many applications as it assumes the existence of a telephone line between the two locations.
The IEC 61334-5-2 defines a method for identifying an unknown phase within a poly phase network by means of injecting a short time stamp signal into the poly phase network when a first reference point, e.g. a zero crossing, occurs in the reference phase voltage. The poly phase network itself serves to communicate the short signal to the location where the unknown phase has to be identified. At the location of the unknown phase, the short signal is extracted from the poly phase power line, and a time interval between the occurrence of the short signal and a reference point, e.g. a zero crossing in the unknown phase voltage is measured. That time interval is then indicative of the phase angle between the reference phase and the unknown phase. The phase angle thus determined allows to identify the unknown wiring phase.
The known methods have in common that reference phase synchronous signals are carried from the location of known phase, herein also called reference location, to the location of unknown phase, herein also called remote location. The remote location compares the received signal phase with the unknown phase to which it is connected, in order to detect its wiring phase. If the phase information is corrupted due to noise or other disturbances, a proper detection of the phase may turn out to be difficult to achieve. Many types of electrical loads connected to the power line tend to generate noise or disturbances in synchronism with the phases to which they are connected. Accordingly, phase synchronous noise signals may interfere in the known method with the short reference phase synchronous signals used for phase detection such that the detection of the reference phase synchronous signals may be impeded. This may become more severe for longer distances between the reference location and the remote location.